Subsistence in itself may sound like complete self-sufficiency. In this technical classification, however, it means not being an attribute inhering in another subject. It does not mean necessary existence or freedom from a cause.
Separate questions:
1. Does this contingent being depend upon God for creation and continuance? Yes.
2. Does it exist in another subject as color does? Not in the case of the entities being classified.
There is therefore no conflict between the doctrine of the world's origination and the division into entities and accidents. One concerns dependence on the Creator; the other, manners of subsistence within creation.